Process of making hydrates of potassium-magnesium phosphates

ABSTRACT

Hydrates of potassium-magnesium phosphates are made by (a) reacting phosphoric acid with an aqueous solution of an aliphatic water-soluble amine wherein the amine is present in an excess sufficient to cause only about one half of the amount of the amine to be used up in the reaction; (b) then dissolving in the solution K2SO4 in an amount per equivalent of P2O5 equal to the stoichiometric amount or up to 5 percent in excess thereof; (c) then pouring an aqueous solution of MgSO4 into the mixture while stirring, the amount of MgSO4 being about 2 equivalents per equivalent of P2O5 and causing precipitation at a pH between about 11.0 and 14.0 and (d) separating the precipitated KMgPO4 hydrate.

United States Patent [72] Inventors Wilhelm Jahn-Held;

Otto Braun, both of Kassel-Wilhelmshohe, Germany [21] Appl. No. 883,556

[22] Filed Dec. 9, 1969 [45] Patented Oct. 26, 1971 [7 3] AssigneeWintershall Aktiengesellschatt Kassel, Germany [54] PROCESS OF MAKINGHYDRATES OF POTASSIUM-MAGNESIUM PHOSPHATES 14 Claims, 2 Drawing Figs.

[52] US. Cl 23/107, 23/105, 71/34 [51] Int. Cl ..C0lb 25/26, C01b 25/30,C01b 25/32 [50] Field of Search 23/106,

[56] References Cited FOREIGN PATENTS 1,265,726 4/1968 Germany 23/105Primary Exam iner-Earl C. Thomas Assistant Examiner-Gregory A. HellerAttorneyMichael S. Striker ABSTRACT: Hydrates of potassium-magnesiumphosphates are made by (a) reacting phosphoric acid with an aqueoussolution of an aliphatic water-soluble amine wherein the amine ispresent in an excess sufficient to cause only about one half of theamount of the amine to be used up in the reaction; (b) then dissolvingin the solution K 50 in an amount per equivalent of P 0 equal to thestoichiometric amount or up to 5 percent in excess thereof; (c) thenpouring an aqueous solution of MgSO into the mixture while stirring, theamount of MgSO being about 2 equivalents per equivalent of P 0 andcausing precipitation at a pH between about 11.0 and 14.0 and (d)separating the precipitated KMgPO hydrate.

PROCESS OF MAKING HYDRATES OF POTASSIUM- MAGNESIUM PHOSPHATES BACKGROUNDOF THE INVENTION The present invention relates to a process for makingpotassium-magnesium phosphate hydrates.

These hydrates have been made previously from KCL and MgCl by usingshort-chain, water-soluble amines such as isobutylamine. If in this casea wet-process phosphoric acid was employed, considerable difficultiesarose in the filtration of the precipitated final product because of thepresence of organic materials and of Ca and F compounds.

There was obtained a slimy precipitate which is hard to filter. Ifsettling agents are employed, the precipitate contains fluorine at anundesirably high rate, for instance l.8 percent fluorine. Since it isnecessary for the precipitation to employ the amines in an excess inorder to obtain an alkaline reaction mass, the economy of this operationdepends to a substantial extent on the recovery of the amine. The amineis driven off from the amine-salt solution by distillation over CaOwhich results in losses up to 10 percent. There are also substantiallosses in K which may be up to 30 percent. Difficulties also exist inseparating the resulting CaCl, solution and disposing of it.

The object of the present invention therefore is to avoid theseshortcomings and to permit the use both of the thermally obtained and ofa wet-process phosphoric acid in an economical way to obtain thehydrates of potassium-magnesium phosphates.

SUMMARY OF THE INVENTION This object is obtained by a process comprisingthe steps of a. reacting thermally or by-wet-process-obtained phosphoricacid with an aqueous solution of an aliphatic primary, secondary ortertiary straight or branched water-soluble amine having from 3 tocarbon atoms, the amine being present in an excess sufficient to causeonly about one half of the amount of amine to be used up in thereaction;

b. then dissolving K 80 in the solution in an amount equal to thestoichiometric amount or up to 5 percent in excess thereof perequivalent of P 0 c. then adding an aqueous solution of MgSO, to themixture while stirring, the amount of MgSO, being about 2 equivalentsper equivalent of P 0 and causing precipitation of the hydrate at a pHbetween about 1 1.0 and 14.0 and d. separating the formed KMgPO, hydrateafter completion of the precipitation.

The novel features which are considered as characteristic for theinvention are set forth in particular in the appended claims. Theinvention itself, however, both as to its construction and its method ofoperation, together with additional objects and advantages thereof, willbe best understood from the following description of the specificembodiments when read in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. la illustrates by way of aflowsheet the making of a hexaor monohydrate of potassium-magnesiumphosphate.

FIG. lb is a similar flowsheet giving the weight parts employed in aspecific example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The amines employed in theinvention preferably have from three to five carbon atoms. Specificexamples of preferred amino compounds are the following: l-aminopropanenpropylamine), Z-aminopropane iso-propylamine), laminobutane,Z-aminobutane, l-amino-2-methyl-propane, laminopentane n-amylamine),l-amino-3(= (=isoamylamine), Z-amino-Z-methylbutane, di-iso-propylamine,din-butylamine and triethylamine.

The addition of the M350. solution to the mixture of phosphoric acid,amine and K,SO preferably is effected by slowly pouring in the MgSO.solution during a period between 2 and 4 hours.

The preferred pH range for the precipitation is between 12.1 and l 1.5.

The mother liquor which remains after precipitation and separation ofthe potassium-magnesium phosphate hydrate is preferably stirred with anequivalent amount of calcium oxide or an excess thereof up to about 20percent so as to precipitate gypsum (CaSO The gypsum may then be washedand the solution of the free amine remaining from the gypsumprecipitation may be recirculated into the process.

The precipitated KMgPO is preferably subjected to a wash, and the washwater is added to the residual mother liquor for reaction with thecalcium oxide and precipitation of gypsum.

The preferred concentration for the amine solution in step (a) of theprocess is between 10 and 30 percent and most preferably between 10 and15 percent. The concentration of the phosphoric acid in the case of wetphosphoric acid during stage (b) is preferably between about 30 and 75percent and in case of thermically obtained phosphoric acid betweenabout 70 and percent. The concentration of the M6650, solution in theprecipitation reaction is preferably between about 10 and 35 percent.

The precipitation and recovery of the hydrate may be effected at roomtemperature, in which case a hexahydrate will be formed. The hexahydratemay then be converted to the monohydrate by calcination at temperaturesfrom about 70 to C.

The precipitation and recovery can also be effected, however, attemperatures above 90 C. in which case a monohydrate will result.

If the amine solution remaining after the precipitation of the gypsum bymeans of calcium oxide is recirculated, it is preferable to remove waterfrom part of the amine solution, preferably from about 5 to 20 percentof the total amine solution. This water removal can be effected bydistillation.

If a hexahydrate is obtained in the main reaction which subsequently isdehydrated to the monohydrate, this has the advantage-that theprecipitation can be carried out at room temperature.

The rapid separation of the precipitate in stage (d) can be accomplishedby decanting the principal amount of the mother liquor as a clearsolution after completion of the precipitation or by siphoning off themother liquor. In case of a continuous operation, a precipitate ispreferably subjected to a concentration (thickening) in a round-bottomsettling tank. The concentrated salt then contains all of the sludgeportion. The salt can be used as a filter aid and thus will shorten thefiltration time.

The purification of the phosphoric acid by precipitation of the silicicacid as silicofluoride and the removal of organic components prior tothe addition of the amine is therefore not necessary in the process ofthe invention.

Because of the use of K SO instead of the conventional MgCl it ispossible to effect the precipitation of gypsum by means of calcium oxide(quicklime) and subsequently to separate the gypsum. The amine thusremains in the mother liquor and does not require distillation, althougha minor proportion thereof may be subjected to such distillation toremove water, as indicated above. All excess amounts of the componentswith K 0, MgO and P 0 in the operation of the invention thus remain inthe mother liquor and do not cause any loss.

It is, however, advisable to avoid using the K 80, in excess of morethan 5 percent of the stoichiometric amount. Otherwise, an amount ofonly 30-70 percent of the 80;, content is precipitated as CaSO and theremaining amount stays in solution. The reason is the formation of KOH.The equilibrium of the reaction between K 80, and CaO to form KOI-l andCaSO, is far to the side of the K 80 Therefore, not all of the S0 isprecipitated as gypsum. In that case, there is not enough free amine inthe recirculated amine solution to neutralize the phosphoric acid andthus the precipitation of the hydrate of KMgPO, is disturbed.

It is therefore desirable to obtain rather complete precipitation orelimination of S0,. This is an important element in using l(,SO insteadof the chlorides heretofore employed. Thus, the replacement of thechlorides by sulfates in the process of the invention is based on adefinitely new approach.

Only if the S0, is practically completely eliminated up to a residualamount of about l g./l. is it possible to use the recirculated aminesolution and thus to carry out the process in a continuous operation,which is a special feature of the present process.

It is also important that, with the process of the invention, the pHvalue must not go down too much. Otherwise, there is an increasingamount of magnesium phosphate obtained which is free of potassium, suchas MgHPO,'3H,O. In that case, the object of the invention could not beobtained.

Because of the contents of a wet-process phosphoric acid in H,SO, or HF,a reduction of the excess of amine may occur. In that case, forinstance, aminosulfate may be formed which uses the pH value of theamine-phosphate solution in the first stage of the invention. In thatcase, a pH of for instance .8 would exist and the precipitation productobtained in stage (c) would only contain l6.7 (80) percent 5 K,O insteadof 26.7% of KMgPO -H,O. In this case, the product would have an Mgcontent of about 28.2 percent while the MgO content of KMg- PO,-H,O isonly 22.9 percent. Such K,0 losses would be unbearable for theindustrial operation of the process.

However, by using an excess of amine in the first stage of the process,which in case of a wet-process phosphoric acid may be up to 100 percentrelative to the phosphoric acid, it is possible to maintain the pH valuefrom beginning to end of the precipitation at the level necessary at theprecipitation stage.

The precipitation product obtained by the invention, after drying, hasthe following composition:

x,o 25.3 mo 22.6 no, 29.: st

x,o 25.5 MgO 24.0 150, 39.0 2.

With the process of the invention it is therefore possible to obtain aprecipitation product of potassium-magnesium phosphate also in acontinuous operation by using solid I(,SO and MgSO, in solution, whichproduct is a close approach to the theoretical value which is asfollows:

|t,o 26.70 M30 22.86 ,0, 40.23 1.

The process of the invention has the definite advantage that evenwet-process phosphoric acid can be used without preceding purificationand that, regardless of the degree of purity of the phosphoric acidemployed, the filtration of the precipitate can be carried out inindustrially acceptable periods of time.

Since, by the process of the invention, it is possible to subject theprecipitated gypsum after separation to a wash and to introduce the washwater into the operation, there is practically no loss in the originallyemployed-materials and in the recirculated amine solution. Thus, thelosses occurring in conventional processes in K,O, MgO and K0, areavoided.

A further advantage of the invention as already briefly mentioned isthat the distillation of the amine may be either avoided entirely orthat only a comparatively small amount, below 20 percent may besubjected to such distillation in order to remove a certain amount fromthe operation.

Thus, the costs of investment and operation for a large distilling plantfor the entire amount of the amine are avoided. it is these costs andthe poor yield of a stoichiometrically composed precipitate whichheretofore prevented an industrially acceptable operation in makinghydrates of potassium-magnesium phosphate.

We claim:

1. The process of making hydrates of potassium-magnesium phosphatecomprising the steps of a. reacting thermally or by-wet-process-obtainedphosphoric acid with an aqueous solution of an aliphatic primary,secondary or tertiary straight or branched water-soluble amine havingfrom three to 10 carbon atoms, the amine being present in an excess, thesaid excess being, in case of the thermically obtained phosphoric acid,up to about 50 percent, and in case of wet-process phosphoric acid, upto about percent;

b. then dissolving K,SO in the solution in an amount equal to thestoichiometric amount or up to 5 percent in excess thereof perequivalent of P,O,;

c. then adding an aqueous solution of MgSO, to the mixture whilestirring, the amount of MgSO being about 2 equivalents per equivalent ofP,O,,, and causing precipitation of the hydrate at a pH between about 11.0 and 14.0 and d. Separating the formed KMgPO, hydrate aftercompletion of the precipitation.

2. The process of claim 1, wherein the water soluble amine has three tofive carbon atoms.

3. The process of claim I, wherein the water-soluble amine is selectedfrom the group consisting of l-aminopropane npropylamine),Z-aminopropane iso-propylamine), laminobutane, 2-arninobutane,l-amino-2-methyl-propane, laminopentane n-amylamine),l-amino-3-methylbutane iso-amylamine), 2-amino-2-methylbutane,di-isopropylamine, di-n-butylamine and triethylamine.

4. The process of claim 1, wherein the addition of the MgSO. solution instep (c) is carried out by slowly pouring the solution into the mixtureof amine solution, phosphoric acid and K,SO during a period of between 2and 4 hours.

5. The process of claim 1, wherein the precipitation of the hydrate iseffected at a pH between about l2.l and l 1.5.

6. The process of claim 1, wherein the following concentrations areemployed:

a. of the amine solution in step (a): about 10 to 30 percent;

b. of the phosphoric acid in step (b): about 30 to 75 percent if awet-process phosphoric acid is used and of about 70 to 95 percent if athermal phosphoric acid is used;

c. of the M0680, solution in step (c): about 10 to 35 percent.

7. The process of claim 1, wherein the precipitation is effected at roomtemperature to obtain a hexahydrate.

8. The process of claim 7, wherein the hexahydrate obtained is subjectedto calcination at about 70 to C. to convert it to the monohydrate.

9. The process of claim 1, wherein the precipitation of the hydrate isefiected at a temperature above 90' C. to obtain a monohydrate.

10. The process of claim I, wherein the mother liquor remaining afterthe precipitation and removal of the KMgPO. is mixed upon agitation withabout an equivalent amount or an excess up to 20 percent of calciumoxide followed by recovery of the precipitated gypsum.

11. The process of claim 10, wherein the hydrate of potassium-magnesiumphosphate obtained in the said precipitation and recovery is subjectedto a wash and the wash liquid is added to the said mother liquor forsaid gypsum precipitation.

prior to said recirculation 14. The process of claim 13, wherein theamount of amine subjected to said distillation is about 5 to 20 percentthe total amine solution.

2. The process of claim 1, wherein the water soluble amine has three tofive carbon atoms.
 3. The process of claim 1, wherein the water-solubleamine is selected from the group consisting of 1-aminopropane (n-propylamine), 2-aminopropane ( iso-propylamine), 1-aminobutane,2-aminobutane, 1-amino-2-methyl-propane, 1-aminopentane ( n-amylamine),1-amino-3-methylbutane ( iso-amylamine), 2-amino-2-methylbutane,di-iso-propylamine, di-n-butylamine and triethylamine.
 4. The proCess ofclaim 1, wherein the addition of the MgSO4 solution in step (c) iscarried out by slowly pouring the solution into the mixture of aminesolution, phosphoric acid and K2SO4 during a period of between 2 and 4hours.
 5. The process of claim 1, wherein the precipitation of thehydrate is effected at a pH between about 12.1 and 11.5.
 6. The processof claim 1, wherein the following concentrations are employed: a. of theamine solution in step (a): about 10 to 30 percent; b. of the phosphoricacid in step (b): : about 30 to 75 percent if a wet-process phosphoricacid is used and of about 70 to 95 percent if a thermal phosphoric acidis used; c. of the MGGSO4 solution in step (c): about 10 to 35 percent.7. The process of claim 1, wherein the precipitation is effected at roomtemperature to obtain a hexahydrate.
 8. The process of claim 7, whereinthe hexahydrate obtained is subjected to calcination at about 70* to120* C. to convert it to the monohydrate.
 9. The process of claim 1,wherein the precipitation of the hydrate is effected at a temperatureabove 90* C. to obtain a monohydrate.
 10. The process of claim 1,wherein the mother liquor remaining after the precipitation and removalof the KMgPO4 is mixed upon agitation with about an equivalent amount oran excess up to 20 percent of calcium oxide followed by recovery of theprecipitated gypsum.
 11. The process of claim 10, wherein the hydrate ofpotassium-magnesium phosphate obtained in the said precipitation andrecovery is subjected to a wash and the wash liquid is added to the saidmother liquor for said gypsum precipitation.
 12. The process of claim10, wherein the amine solution remaining after precipitation of thegypsum is recirculated into the process.
 13. The process of claim 12,wherein part of the amine solution remaining after the precipitation andseparation of the gypsum is subjected to distillation to remove watertherefrom prior to said recirculation.
 14. The process of claim 13,wherein the amount of amine subjected to said distillation is about 5 to20 percent the total amine solution.